Igto] HOWE—REFORESTATION OF SAND PLAINS 145 
white birch by white pine. It will be noted that the numbers of white 
birch on the unit area for the ages given are 245, 105, 68, and 16 
and that the numbers of white pine increase as follows: 0, 5, 18, 46 
This illustrates the most striking fact in the course of succession on 
worn-out pastures, that is, gradual replacement of white birch by 
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IG, eras plot 17. White birch 25 years old, 5 meters square, 50™ fr 
white pine; under layer Polytrichum and Rubus hispidus; B, Betula populifolia 6 
* PS, Pinus Strobus 16; QV, Quercus velutina 6; M, Myrica asplenifolia 14; S, Spiraea 
tomentosa 3; A, Acer rubrum 1. 
white pine. If a stand of white birch is within proper distance 
from white pine seed trees, the birch acts as a nurse tree to the pine. 
In the clearing of the sand plains occasional white pine trees were 
left along the margins of wood lots and along highways. Such trees 
became the originators of numerous pure stands of white pine in the 
